I had an '82 300SD that had similar trans problems. I used the Gunk additive as well with similar results. I was told repeatedly the problem was hard seals. I found that as the trans heated up from driving it got better.

Question: Did you drain the torque converter as well?

Valvoline now has a trans additive that helps "condition" older seals. I would remove enough ATF and add a quart. It might return the flexibilty of the seals.

Additionally, I was encouraged to check-out the vacuum modulator and the lines to it. Have you?